System for the Characterization of Emissive Elements

ABSTRACT

A method is provided for the selective harvest of microLED devices from a carrier substrate. Defect regions are predetermined that include a plurality of adjacent defective microLED devices on a carrier substrate. A solvent-resistant binding material is formed overlying the predetermined defect regions and exposed adhesive is dissolved with an adhesive dissolving solvent. Non-defective microLED devices located outside the predetermined defect regions are separated from the carrier substrate while adhesive attachment is maintained between the microLED devices inside the predetermined defect regions and the carrier substrate. Methods are also provided for the dispersal of microLED devices on an emissive display panel by initially optically measuring a suspension of microLEDs to determine suspension homogeneity and calculate the number of microLEDs per unit volume. If the number of harvested microLED devices in the suspension is known, a calculation can be made of the number of microLED devices per unit of suspension volume.

RELATED APPLICATIONS

Any and all applications, if any, for which a foreign or domesticpriority claim is identified in the Application Data Sheet of thepresent application are hereby incorporated by reference under 37 CFR1.57.

BACKGROUND OF THE INVENTION 1. Field of the Invention

This invention generally relates to light emitting diodes (LEDs) havinga size of less than 100 microns and, more particularly, to microLEDfabrication processes and systems.

2. Description of the Related Art

A red/green/blue (RGB) display is composed of pixels that emit light atthree wavelengths corresponding to the visible colors red, green, andblue. The RGB components of the pixel, each of which is referred to as asub-pixel, are energized in a systematic way to additively produce thecolors of the visible spectrum. There are several display types thatproduce the RGB images in different ways. Liquid crystal displays (LCD)are the most prevalent technology and they produce RGB images by shininga white light source, typically a phosphor produced white LED, through acolor filter of a subpixel. Some portions of the white light wavelengthsare absorbed and some transmitted through the color filter. As aconsequence, the efficiency of an LCD display may be less than 4% andthe contrast ratio is limited by light leaking through the cell. Organiclight emitting diode (OLED) displays produce RGB light by directemission of each of those wavelengths of light at a pixel level withinthe organic light emitting material. The OLED materials are directemitting so the display contrast ratio is high, but the organicmaterials can be subject to long term degradation causing image burn-in.

A third display technology and the one addressed herein is the microLEDdisplay, which uses micro-sized (5 to 100 microns (μm) diameter)inorganic LEDs for direct emission of light at the subpixel level.Inorganic microLED displays have several advantages over competingdisplays. Compared with LCD displays, the microLED display has very highcontrast over 50,000:1 and higher efficiency. Unlike the OLED display,inorganic LEDs do not suffer burn-in effects and the achievablebrightness is significantly higher.

MicroLEDs are fabricated from metalorganic chemical vapor deposition(MOCVD) wafers like those used to make LEDs for general lighting, whichmakes the cost per device very low but also introduces some problemsthat are unique to the microLED technology. The structures used forfluidic assembly of microLEDs have been exhaustively described in parentU.S. Pat. No. 10,643,981, which is incorporated herein by reference. Foruse in general lighting the most important characteristic of a device islow cost per generated photon, to minimize the cost of each light bulb.That constraint has caused LED fabrication practices to use a processcalled binning to deal with process variability and defects. Brieflystated, the binning process consists of testing all LEDs after packagingand placing each device in a comparable group with similar efficiencyand emission wavelength characteristics, while defective devices arediscarded. The binning process allows the MOCVD fabrication to becheaper because defect reduction and process control methods and costsare minimized.

A recent characterization of 40 μm microLEDs fabricated from typicalgallium nitride (GaN) based MOCVD wafers showed that 0.25% of deviceswere shorted and 0.75% were open. These defects would cause a darksub-pixel, which is not acceptable for a display product. The microLEDis not packaged and the very small size of the device, especially theelectrodes, makes device handling and functional test difficult. Becausean ultrahigh definition (UHD) display requires at least 24.8 millionmicroLEDs (3×3840×2160) the testing times become astronomical.Therefore, the binning technique is not practical to identify anddiscard defective microLEDs. Consequently, new structures and methodsare required to prevent defective microLEDs from creating defectivesubpixels. It is possible to remove a defective microLED and replace itas has been described in parent applications Ser. Nos. 16/125,671,16/595,623, and 16/693,674, which are incorporated herein by reference,but the mechanical pick-and-place tools are expensive to buy andoperate. It would be more desirable to identify defective microLEDs andprevent them from entering the suspension used for fluidic assembly.

LEDs used for general lighting are much larger than those used formicroLED displays (up to 3-4 millimeters (mm) per side versus 5 to 100μm in diameter) so the patterning and electrode requirements aresignificantly different. MicroLEDs are bonded to the substrateelectrodes using either a solder material or an asymmetric conductivefilm (ACF), while large general lighting LEDs are often connected bywire bonding or solder paste on a lead frame. Because the microLEDs arequite small, the techniques for handling devices and especially forassembling microLED displays are quite different from those which havebeen developed for the very large LEDs used in general lighting.

To fabricate a microLED display, green and blue GaN microLEDs arefabricated on a sapphire substrate and red aluminum gallium arsenicphosphide (AlGaAsP) microLEDs are fabricated on a GaAs substrate. Afterfabrication and segmentation, the microLEDs must be transferred to asecond substrate that becomes the emissive display. The second substratecan be a silicon (Si) wafer (or chip) with built-in control circuitry,or it can be a substrate of glass or flexible plastic with thin filmtransistors. The conventional method of transfer is a mechanicalpick-and-place system, which uses a pickup head to capture a device andposition it on the display substrate. Other mechanical transfer methodsthat use a stamp or the like to transfer a block of microLEDs at thesame time are referred to as mass transfer. An alternate technology, asdescribed herein, uses a fluidic assembly process to position themicroLEDs.

Briefly stated, the fluidic assembly process applies a liquid suspensionof microLEDs to a substrate with an array of trap sites (wells) andmoves the suspension to cause the microLEDs to be assembled in the trapsites. For fluidic assembly to succeed, it is necessary to harvestmicroLEDs from the growth substrate without including defects, formulatea suspension with a known concentration of microLEDs, and then dispensethe suspension uniformly over the display substrate.

The handling of micron-scale particle suspensions is well establishedfor systems like cell cultures in the biological sciences or abrasiveslurry in industrial applications. In all cases, the objective of thesuspension handling system is to achieve a highly homogeneous suspensionand transfer the suspension to the target process with a high degree ofcontrol over volume and concentration. Suspension uniformity isgenerally achieved through direct mechanical mixing with a submergedimpeller, or by active circulation via pump. Transfer of the well-mixedsuspension to a target process is generally done via tubing with pumpingdownstream of the supply tank, or by pressurizing a sealed tank.Volumetric control for well-suspended systems is achieved by controllingflow rates with pressure differentials and metering net flow with valvetiming. It may be desirable to control the suspension concentration,especially when the suspension is being reused, and therefore it iscommon for transfer tubing to include fittings with multiple inputs sothat neat carrier fluid may be balanced with the suspension.

Unfortunately, the conventional techniques for suspension handling arenot compatible with the properties of microLEDs or the requirements ofthe fluidic assembly technology. Specifically, the microLED suspensionshave distinguishing characteristics described below that necessitate thedevelopment of an alternative approach.

FIG. 1 depicts a microLED suspension with a uniform distribution afteragitation that settles to about half the liquid column height after timet1, and is completely settled after time t2. Unlike abrasive slurries,which are formulated to extend homogeneous mixing and have settlingtimes measured in months, microLED suspensions are formulated to have arelatively short settling time. MicroLEDs must settle to the targetsubstrate surface for assembly to take place, so microLED suspensionsfor fluidic assembly generally settle completely in minutes and loseuniformity very quickly after cessation of mixing. As an example, thedisk shaped microLEDs 42 μm in diameter by 5 μm thick, fabricatedfollowing the center mesa design presented in parent application Ser.No. 16/406,080 (incorporated herein by reference), have a hydrodynamicdiameter of 18.9 μm. Objects of this size have a terminal velocity inliquid when the force of gravity is balanced by the viscosity of theliquid according to the following equation:

$v_{T} = \frac{g{D_{LED}^{2}( {\rho_{LED} - \rho} )}}{18\mu}$

where D_(LED) is the hydrodynamic diameter, ρ is the liquid density,ρ_(LED) is the microLED density, and μ is the liquid viscosity. Forwater, the terminal velocity is 1.1 millimeters per second (mm/sec), soin a typical container like a 50 milliliter (ml) Falcon tube themicroLEDs settle completely in a minute or so.

MicroLEDs generally have surfaces that include metal, inorganic, andorganic materials. As such, it is nearly impossible to prevent temporarystiction to solid surfaces in contact with the microLED suspension. As aconsequence, containers holding microLED suspensions are typicallyfabricated from hydrophobic materials such as acetal homopolymer,polytetrafluoroethylene (PTFE), polypropylene, and the like to minimizestickiness. The final state in FIG. 1 shows the effect of theinteraction between microLEDs and the wall of the container where thebottom cone is less than the angle of repose, so some microLEDs stick tothe walls of the vessel and do not settle to a uniform layer at thebottom.

Microbes in biological applications are generally sturdy enough tosurvive internal mixing (such as with a stir bar) without lysing, andindustrial abrasive suspensions such as chemical mechanical polish (CMP)slurries are suspended by drum circulation or impeller mixing withoutdamage. In contrast, microLEDs are friable and can be fractured fromdirect mechanical mixing or pumping. A broken microLED is similar inmajor dimension to a good microLED, so it cannot be removed from thesuspension by filtering, and it interferes with fluidic assembly bypartially blocking a trap site.

MicroLEDs represent a large fraction of the cost to manufacture adisplay, and inefficiencies in μLED utilization and recapture stronglyinfluence cost. Contrasted to biological and industrial abrasiveapplications, the components in suspension are significantly morevaluable.

Unlike conventional suspensions, the performance characteristics of eachindividual microLED are important because each device makes up onesub-pixel. It is necessary to strictly control the population ofmicroLEDs available for assembly to control the emission distribution ofthe completed display. As such, suspension handling must be designed toprevent cross-contamination.

MicroLED properties and the stringent requirements for displayfabrication rule out the conventional industrial systems and methods ofsuspension handling. Controlled and efficient dispense of clean,high-quality components is critical to fluidic assembly because theforces involved in fluidic assembly are limited by the threshold atwhich assembled components become detrapped. Rapid fluidic assemblythen, relies on short travel paths on the substrate between microLEDsand their eventual assembly (trap) site. Optimal dispense of themicroLED suspension onto the display substrate must therefore not onlybe low-loss and damage free, but also fast and highly uniform.

It would be advantageous if harvest and dispersal methods existed thatwere specifically tailored to the handling of inorganic microLEDs usedin fluidic assembly.

SUMMARY OF THE INVENTION

Described herein are systems and methods for formulating andmanipulating a micro-light emitting diode (microLED) suspension that aresuitable for the fluidic assembly of microLED displays. A selectiveharvest method produces a microLED suspension composed of known goodLEDs at a well determined concentration in an appropriate liquid. ThemicroLED suspension is supplied to the display substrate using adispensing system which minimizes damage and loss of microLEDs whileproducing a uniform distribution of devices at a controlled density overthe display substrate. This optimum initial condition is essential forsuccessful fluidic assembly of microLED displays.

Accordingly, a method is provided for the selective harvest of microLEDdevices from a carrier substrate. The method provides inorganic microLEDdevices attached to a carrier substrate with an adhesive. Defect regionsare predetermined (e.g., an edge bead) that include a plurality ofadjacent defective microLED devices. A solvent-resistant bindingmaterial is formed overlying the predetermined defect regions andexposed adhesive is dissolved with an adhesive dissolving solvent. Someexamples of an adhesive dissolving solvent include acetone, toluene,trichloroethane, N-methylpyrrolidone (NMP), xylene, cyclohexanone, butylacetate, or combinations thereof.

Non-defective microLED devices located outside the predetermined defectregions are separated from the carrier substrate while adhesiveattachment is maintained between the microLED devices inside thepredetermined defect regions and the carrier substrate. In response toseparating the microLED devices from the carrier substrate, functionalmicroLED devices are collected in a harvesting container. In onevariation, only certain sections of the carrier substrate are exposed tothe adhesive dissolving solvent, so that microLEDs are only separatedfrom the selectively exposed sections of the carrier substrate.

Further, the carrier substrate may be inspected to locate defectivemicroLED devices in non-predetermined defect regions, and thesolvent-resistant binding material may also be formed overlying thesenon-predetermined defect regions. In one aspect, inspection may locatenon-predetermined solitary defective microLED devices, and a lasertrimming process may be used to eject the solitary defective microLEDdevices. The inspection may be carried out using optical comparison,electroluminescence, photoluminescence, or cathodoluminesence testing.

The microLED devices collected in the harvesting container are typicallya suspension of functional microLED devices having an averagecross-sectional dimension s. However, impurities also exist in thesuspension fluid. In one aspect, a filtering process using a mechanicalmesh, elution, fractionation, or combinations thereof, is performed toremove impurities having a maximum cross-sectional dimension greaterthan t, where t>s. Likewise, a separate filtering process may removeimpurities having a maximum cross-sectional dimension less than p, wherep<s. In one aspect prior to filtering, the adhesive dissolving solventin the harvested microLED suspension is replaced with a filteringsolution having a lower viscosity than the adhesive dissolving solvent.Alternatively, or after filtering, the fluid in the harvested microLEDsuspension can be replaced with an assembly solution having either alower polarity or a higher evaporation rate. In one aspect, a surfactantmay be added such as anionic, cationic, non-ionic surfactants, orcombinations thereof.

A method is also provided for the dispersal of microLED devices on anemissive display panel. The above-described suspension of harvestedmicroLED devices is transferred to a transparent first container andagitated. Some examples of an agitation process include externalvibration of the first container, creating a fluid flow in thesuspension, and the flowing a gas through the first container. Thesuspension opacity is optically measured at a plurality of firstcontainer heights to determine suspension homogeneity. When thedetermined homogeneity is greater than a homogeneity minimum threshold,the suspension can be dispersed on the top surface of an emissivedisplay panel. Some examples of dispersion processes include single-stepmass decantation, multi-step pipette translation, nozzle limitedcontainer translation, and translating tube.

If the number of harvested microLED devices in the suspension is known,a calculation can be made of the number of microLED devices per unit ofsuspension volume. As a result, a known first number of microLED devicescan be deposited on the emissive display panel in response to dispersinga first volume of suspension. Advantageously, after determining thenumber of assembly sites in a first region of the emissive display paneltop surface, the deposited known first number of microLED devices is atleast equal to the number of assembly sites in the first region.

The optical measurement of the suspension opacity is performed byarranging a plurality of light emitting devices having predeterminedoutput light intensity, directed towards a center axis of the firstcontainer, and spaced a first predetermined distance from each otheralong a first vertical axis. A plurality of light detection devices arespaced the first predetermined distance from each other along a secondvertical axis, with each light detection device having an input directedtowards a corresponding light emitting device output. Then, a comparisonis made of the intensities of light received by the light detectiondevices.

In one aspect, in response to determining the suspension opacity at aplurality of first container heights, a first number of microLED devicesper unit of suspension volume can be calculated. Subsequent todispersing an aliquot volume of suspension onto an emissive displaypanel top surface, the optical measurements can be repeated to calculatea second number of microLED devices per unit of suspension volume. If aknown aliquot volume of suspension is transferred to a second containerand a predetermined amount of fluid is added to (or removed from) thesecond container, it is possible to calculate a third number of microLEDdevices per unit of suspension in the second container. If fluid isadded to (or removed from) the suspension in the first container, thensubsequent to agitating the suspension, the suspension density can beoptically measured again to calculate a fourth number of microLEDdevices per unit of suspension volume.

Additional details of the above-described methods, as well as systemsfor segregating regions of a microLED carrier substrate, harvestingmicroLEDs, characterizing a microLED suspension, are presented below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a microLED suspension with a uniform distribution afteragitation that settles to about half the liquid column height after timet1, and is completely settled after time t2.

FIG. 2 is a partial cross-sectional view of a system for characterizinga micro-light emitting diode (microLED) suspension.

FIGS. 3A and 3B are partial cross-sectional schematic views ofcomponents in a system for selectively harvesting microLED devices froma carrier substrate.

FIG. 4 is a partial cross-sectional view of a system for selectivelysegregating regions of a microLED carrier substrate.

FIG. 5 is a schematic drawing depicting the preparation of a microLEDpopulation for fluidic assembly as consisting of three successive steps.

FIG. 6 is a partial cross-sectional view of a typical microLED on acarrier wafer after device processing is complete.

FIGS. 7A and 7B depict wafer maps of known defect locations frommicroLED processing (FIG. 7A) and typical alignment structures (FIG.7B).

FIG. 8 is a graph depicting GaN microLED cathodoluminesence spectra.

FIG. 9 is a composite defect map that can be used to guide the defectcontrol processes.

FIG. 10 is a plan view of a wafer where emission wavelength is shown asequal width contours.

FIGS. 11A and 11B are partial cross-sectional views respectivelydepicting some representative microLED defects and corrective measures.

FIGS. 12A, 12B, and 12C are partial cross-sectional views of a microLEDharvest in solvent.

FIG. 13 is a partial cross-sectional view depicting an appropriatestorage container.

FIG. 14 is a graph showing the major diameter of exemplary microLEDs andcontaminants.

FIG. 15 is a schematic drawing of an elution cell.

FIG. 16 is a schematic drawing of a continuous flow fractionationfiltering method.

FIGS. 17A through 17C are, respectively, a graph depicting exemplarymeasurements of optical transmittance versus time for a suspension of 42μm diameter microLEDs in 20 mL of isopropanol (IPA), a graph depictingexemplary measurements of optical transmittance versus time for 1.3million microLEDs in different IPA volumes, and a calibration curve formicroLED suspension concentration versus optical transmittance for thissystem.

FIGS. 18A and 18B contrast dispense density gradients, respectively, forsingle and two pass/double speed dispensing paths.

FIGS. 19A through 19C are schematics respectively depicting the directtransfer of a well-mixed suspension from the initial source container tothe assembly substrate via decanting from container, nozzle, and tubing.

FIG. 20 is a schematic showing controlled volume pipetting from awell-mixed suspension to the assembly substrate.

FIG. 21 is a schematic showing amended suspension in an intermediatepot, and subsequent aspiration and dispense via a bubble mixed dispensehead.

FIG. 22 is a schematic of a parallel dispense method using an array ofdispense heads.

FIG. 23 is a flowchart illustrating a method for the selective harvestof microLED devices from a carrier substrate.

FIGS. 24A through 24C are flowcharts illustrating a first method for thedispersal of microLED devices on an emissive display panel.

FIG. 25 is a flowchart illustrating a second method for the dispersal ofmicroLED devices on an emissive display panel.

DETAILED DESCRIPTION

FIG. 2 is a partial cross-sectional view of a system for characterizinga micro-light emitting diode (microLED) suspension. The system 200comprises a transparent container 201 having a vertical center axis 202.A plurality of light emitting devices 204 a through 204 n (LED array) isshown with each light emitting device having a predetermined outputlight intensity, directed towards the center axis 202 of the container200 and spaced a first predetermined distance 206 from each other alonga vertical first axis 208 parallel to the center axis 202, where (n) isan integer greater than 1. A plurality of light detection devices 210 athrough 210 n (photodetector array) is spaced the first predetermineddistance 206 from each other along a vertical second axis 212 parallelto the center axis 202. The light detection devices 210 a -210 n eachhave an optical input directed towards a corresponding light emittingdevice output and an output, respectively on lines 214 a through 214 n ,to provide an electrical optical density signal responsive to measuredlight intensity. A monitoring device 216 has an input to accept theoptical density signals 214 a -214 n . The monitoring device 216compares the intensities of light associated with the optical densitysignals and supplies an output on line 218 responsive to the comparison.In the interest of simplicity, the figure shows the number of lightemitting devices equal to the number of light detection devices, withidentical spacings. Further, the light emitting devices output the sameintensity of light. However, it should be understood that oncecalibrated, a similar system may be enabled without these explicitlimitations.

The container 201 includes a suspension 218 of microLEDs. The monitoringdevice 216 is able to supply either a microLED homogeneity measurement,or a calculation of the microLED count per unit volume of suspension online 218 as determined from the homogeneity (density) measurement. Inone aspect, the monitoring device 216 includes a non-transitory memory220 with a stored calibration curve 222. In this case, the monitoringdevice 216 is able to supply a microLED count per unit volume ofsuspension on line 218 in response to comparing the optical densitysignals on lines 214 a -214 n to the calibration curve 222. As acomponent of the calibration curve, in one aspect the monitoring device216 is able to receive and store data concerning the volume of thecontainer 201.

In another aspect, the monitoring device 216 has an input on line 224 toaccept a calibration input signal representing the total number ofmicroLEDs in the suspension, in which case the monitoring device is ableto supply a microLED count per unit volume of suspension in response tocomparing the optical density signals 214 a -214 n to the total numberof microLEDs. The total number of microLEDs may be known, for example,by taking a count of the number of functional microLEDs harvested from acarrier substrate. The input on line 224 may alternatively, or inaddition, accept a running measurement of suspension volume.

In one variation, the monitoring device 216 accepts sets of opticaldensity signals 214 a -214 n collected over a period of time andsupplies an output on line 218 of either microLED settling time ormicroLED size. Further, the container 200 may be divided by a pluralityof graduations 226 a through 226n and may include a homogeneoussuspension of microLEDs (the suspension 218 shown is not homogeneous).In this case, the light detection devices 210 a -210 n detect changes inthe level of suspension as measured against the container graduations226 a -226 n. The monitoring device supplies an output on line 218 ofeither the number of microLED devices dispersed from the container 200or a volume of suspension dispersed from the container. Advantageously,this output can be supplied in real-time.

To aid in the above-described measurements, an agitation means may beused to homogenize the suspension. A number of homogenization mechanismsare described in more detail below. In one aspect as shown, a solution(solvent) or gas can be used to mix the suspension. Knowing thesuspension volume, the agitation mechanism can be tuned for optimizedmixing.

FIGS. 3A and 3B are partial cross-sectional schematic views ofcomponents in a system for selectively harvesting microLED devices froma carrier substrate. As shown in FIG. 3A, the system 300 comprises aturntable or vacuum chuck 302 having a rotating (spindle) interface formounting a carrier substrate 304 comprising inorganic microLED devices306 attached to the carrier substrate with an adhesive 308. An elbow 310is connected to the rotating vacuum chuck 302, with a plurality ofselectable settings for determining an angle (tilt) 312 of turntablerotation in an xz-axes plane. A gantry 314 is connected to the elbow310, with a plurality of selectable settings for determining the heightof the turntable along a z-axis. A tray 316 includes an adhesivedissolving solvent 318 and has a top opening to accept the carriersubstrate 304. A controller 320 has outputs connected to the gantry 314and elbow 310, respectively on lines 322 and 324, supply the height andangle settings. As shown, the system 300 permits selected radialsections of the carrier substrate 304 to be exposed to the adhesivedissolving solvent 318 in response to the gantry 314 and elbow 310settings. MicroLED devices 306, separated from the selectively exposedsections of the carrier substrate 304, are harvested in the tray 316. Inthis example the Ai region of the carrier substrate is being harvested(see FIG. 10)

In one aspect, the controller 320 has an input on line 326 to accept afirst map of microLED performance regions, and supplies gantry 314 andelbow 310 settings selecting radial regions of the carrier substrate 304for exposure to adhesive dissolving solvent, in response to the firstmap.

As shown in FIG. 3B, the system 300 may comprise an inspection subsystem328 having an optical input 330 and an output on line 332 connected tothe controller 320 for the identification of individual defectivemicroLED devices 306 on the carrier substrate 304. A trimming laser 334has an input on line 336 connected to the controller 320 to accept asecond map of defective microLED devices and an output 338 to ejectthrough radiation, defective microLED devices from the carrier substrate304 in response to the second map.

FIG. 4 is a partial cross-sectional view of a system for selectivelysegregating regions of a microLED carrier substrate. The system 400comprises a controller 402 that has an output on line 404 to supply afirst map of a predetermined defect region 406 on a carrier substrate408. A printer 410 has an input on line 404 to accept the first map anda nozzle 412 to apply a solvent-resistant binding material 414 toselected regions of the carrier substrate 408 in response to the firstmap. MicroLEDs 415 in the selected region 406 remain attached to thecarrier substrate 408 despite exposure to an adhesive dissolving solvent(not shown). Some examples of solvent-resistant binding material 414include SU-8, epoxy resin, polyethylene terephthalate (PET),acrylonitrile butadiene styrene (ABS), and polyimide.

Optionally, system 400 may further comprise an inspection subsystem 416having an optical input 418 and an output connected to the controller online 420 identifying a non-predetermined defective microLED deviceregion 422 on the carrier substrate 408. The printer input on line 404is able to accept a second map of the non-predetermined defectivemicroLED device region 422 from the controller 402 and apply thesolvent-resistant binding material to detected defective microLED deviceregion 422 (binding material not yet applied in the figure) in responseto the second map.

FIG. 5 is a schematic drawing depicting the preparation of a microLEDpopulation for fluidic assembly as consisting of three successive steps.First, the incoming microLED wafers are inspected to determine thelocations of defective microLEDs. Based on the defect map, defectivemicroLEDs and other debris are either removed from the carrier substrateor encapsulated to prevent them from being harvested into thesuspension. The carrier wafer is then immersed in a solvent to dissolvethe adhesive holding the known good microLEDs onto the substrate and themicroLEDs are rinsed into a holding container. The solvent withdissolved adhesive is carefully decanted after allowing the microLEDs tosettle and several solvent exchanges are carried out to remove theremaining adhesive residue. The resulting suspension is filtered toremove particles with a size significantly different than the microLEDs,and the amount of solvent is adjusted so that the density of microLEDsin suspension is appropriate for subsequent mixing and dispenseoperations.

Using the suspension of microLEDs, three alternate dispensing systemsthat take different approaches with varying tradeoffs for dispensespeed, volume control, and complexity can be used to apply microLEDsuspension to the display substrate. In each case several separatealiquot transfers are made to cover the substrate. An aliquot transfermay either be directly to the substrate or through a controlled volumeintermediate in the form of the ‘Ink Pot’ that enables both the dilutingof the suspension and the use of a dispense head actively mixing themicroLED suspension prior to dispense onto the substrate. Forsufficiently long-settling time suspensions, the direct transfer ispreferred. Finally, the substrate can be inspected for uniformity andadditional small area dispenses can be used to fill in low densityareas.

FIG. 6 is a partial cross-sectional view of a typical microLED on acarrier wafer after device processing is complete. MicroLEDs aretypically fabricated on sapphire substrates and transferred to a carrierwafer using laser lift off (LLO) as described in parent patent U.S. Pat.No. 10,643,981, which is incorporated herein by reference. The resultingwafer has millions of microLEDs embedded in an adhesive layer on acarrier wafer as shown. Unfortunately, the wafer will also have severaldifferent defect types (including process control structures) thatadversely affect subsequent processing, so selective harvest is used toensure that the fluidic assembly process can be carried out using onlyknown good microLEDs.

FIGS. 7A and 7B depict wafer maps of known defect locations frommicroLED processing (FIG. 7A) and typical alignment structures (FIG.7B). The device processing to fabricate microLEDs has several knowndefects or process control structures that occur systematically in thesame location because of the nature of the process as shown. Each waferhas an identifying mark scribed near the edge of the wafer, whichcreates pits in the substrate surface that disrupt the microLED pattern.Each of the several processing steps that form the microLED structureuses a photoresist coating to transfer a pattern to the wafer.Photoresist coating is not perfect, so the edge of the wafer has patterndefects in a ring shape known as the edge bead. The photolithographyprocess uses a series of marks to locate each layer with respect toprevious layers. The resulting alignment marks and metrology structures(FIG. 7B) are necessary for fabrication but are defects if included insolution with the harvested microLEDs. These defective areas are muchlarger than the typical microLED size and the edge bead exclusion ringmay be as wide as 2-3 mm. The first component of a wafer defect map isthe size and location of these large systematically placed structures.

A second class of defects are random processing defects such as chemicalmechanical polish (CMP) scratches on the substrate, large residualgallium nitride (GaN) blocks caused by fall-on particles in isolationetch, missing metal electrodes, and the like. These larger defects canbe identified by optical scanning, which compares adjacent images fordifferences that do not match the expected microLED pattern. Thiscomponent of the defect map consists of a series of coordinatesoutlining each defective area and the size of the defect.

The most important class of defects is functional defects that affectthe electrical properties and optical emission of a microLED. Mappingthese defects can be accomplished by four different complimentarytechniques.

1) Perhaps the most desirable technique is an electroluminescence (EL)test that probes each microLED and measures the resulting emission. Thistest directly identifies weak devices with low emission, as well asshorted or open devices. The disadvantage is that the technique is slowand it is difficult to probe the small electrodes, especially withoutdamaging the electrode surface. It is possible that this technique canbe used to measure a few representative devices and a region of a wafercan be added to the defect map because of low fluence or the wavelengthof emission being off target.

2) Micro photoluminescence (PL) applies a light source, typically alaser, at a wavelength that excites transitions in the LED structure andmeasures the wavelength and intensity of the resulting emission. Thistechnique can identify metalorganic chemical vapor deposition (MOCVD)defects, as well as cracked or shorted devices, but it cannot identifymissing metal or open contacts.

3) With optical comparison the usual method is to compare two images andlook for differences between them, with the difference being a defect.An optical image can also be compared to a pattern (the-to-database).

4) Cathodoluminescence (CL) is described below.

FIG. 8 is a graph depicting GaN microLED cathodoluminesence spectra.Micro cathodoluminescence uses an electron beam from a scanning electronmicroscope (SEM) to excite transitions in the LED structure and measuresthe wavelength and intensity of the resulting optical emission. GaNmicroLEDs have multiple characteristic emission lines as shown, sodifferences in spectra can identify different defect mechanisms. A weakor absent emission peak at 455 nm indicates a shorted LED or a problemwith the MQW. Higher defect emission in the broad peak around 570 nm canindicate etch damage or poor quality of the n-GaN. Low intensity of theexciton peak can also indicate poor quality of the initial growth ormissing dopant. Any serious deviation from a typical microLED spectrumprobably indicates a device that will not perform well in a display.

FIG. 9 is a composite defect map that can be used to guide the defectcontrol processes. The functional tests identify individual microLEDswith defects, so this portion of the defect map consists of the X-Ycoordinates of each defective LED. The electroluminescence andphotoluminescence mapping techniques can also produce a map of the LEDperformance characteristics including emission wavelength, efficiency,and threshold voltage. Using these techniques, it is possible toidentify regions with different performance so the selective harvesttechnique can be adapted to harvest smaller regions of the wafer toproduce a microLED suspension with a tighter distribution of uniformityin device performance.

FIG. 10 is a plan view of a wafer where emission wavelength is shown asequal width contours. In some cases it is desirable to harvest eachradial band of the carrier substrate separately, so that separatesuspensions (e.g., four bands as shown) each have a narrow wavelength orefficiency distribution. This simple binning technique can be used toproduce displays with improved color mura by combining suspensions fromseparate wafers that have the same emission wavelength.

FIGS. 11A and 11B are partial cross-sectional views respectivelydepicting some representative microLED defects and corrective measures.Based on the size information in the composite defect map, the wafer isprocessed to eliminate defects in two ways as shown in FIG. 11B. Largedefect areas, such as the edge bead and areas of missing pattern, arecoated with a material that is resistant to the solvents used in theharvest process, so the defective regions remain captured on the carrierwafer after harvest. Suitable materials are SU8, epoxy resin,polyethylene terephthalate (PET), acrylonitrile butadiene styrene (ABS),or polyimide, which can be sprayed, patterned by inkjet, or even appliedwith a brush or stylus. The wafer may be baked after treatment to curethe holding material. The smaller defects identified as single devicesare removed from the harvest wafer using a pulsed laser beam that isfocused to a spot size smaller than the diameter of the microLED. Thelaser wavelength and energy are chosen so that absorption in thedefective microLED causes rapid heating which ejects the target devicefrom the adhesive layer. An auxiliary vacuum nozzle adjacent to thelaser target zone is used to capture and dispose of the ejected devicesto prevent redeposition on the carrier wafer.

FIGS. 12A, 12B, and 12C are partial cross-sectional views of a microLEDharvest in solvent. In FIG. 12A a carrier wafer with the known goodmicroLEDs is placed in a solvent resistant container and immersed in asolvent or solvent mixture that dissolves the adhesive that is holdingthe microLEDs. The solvent bath may be acetone, toluene,trichloroethane, N-Methylpyrrolidone (NMP), xylene, cyclohexanone, butylacetate, or the like, and the solvent may be heated and gently agitatedto speed up the harvest process. Care is taken to prevent mechanicalcontact between the container and the microLEDs on the carrier, whichcan damage the devices. A container which supports the carrier wafer ina vertical position works well to allow solvent circulation, and permitsmicroLEDs to settle to the bottom of the container as the adhesivedissolves.

An alternate harvest method (FIG. 12B) places the carrier waferhorizontally in a shallow container with the carrier supported by anarrow ledge that contacts the carrier only in the edge bead region asshown. It can be seen that the printed capture material protects theadhesive from solvent so the captured regions above a certain sizeremain on the carrier wafer. If the carrier substrate is not promptlyremoved from the solvent bath it is possible for large pieces of capturemedia to be released as the adhesive is undercut. When all of themicroLEDs have been released and settled to the bottom of the container,the carrier wafer is removed, inspected for residual good microLEDs anddiscarded.

The “complete” wafer harvest methods described above are fast and simplebut there are cases where selective harvest techniques can be used.MicroLEDs are harvested only in regions that are in contact with theadhesive dissolving solvent so a simple selective harvest can be carriedout by placing a small droplet on the horizontal carrier wafer as shownin FIG. 12C. After the adhesive is dissolved, the harvested area isrinsed to remove the released microLEDs from the carrier wafer.

Another system for selective harvest of radial regions of a wafer usesthe same principle of exposing a controlled region of the carrier waferto the adhesive dissolving solvent, and is depicted in the system ofFIG. 3A. Based on the wavelength contour map of FIG. 10 for example, thetilt and z height of the carrier substrate are adjusted so that only theregion λ₁ (see FIG. 10) is submerged in the solvent bath. The carriersubstrate is slowly rotated to equally expose the radial band to thesolvent and the released microLEDs settle to the bottom of the solventtank. When the first region has been harvested, the carrier is withdrawnfrom the solvent and the microLEDs with the Xi wavelength distributionare removed from the solvent bath. Then the carrier can be repositionedto harvest the λ₂ region and the process proceeds in the same way forsuccessive harvest regions.

Typically, the harvest solvent is heavily contaminated with adhesiveresidue which also coats the microLEDs, so the microLED suspension isprocessed through a series of solvent exchanges to remove thecontaminants. One solvent exchange cycle proceeds as follows:

-   -   1) The solvent is agitated indirectly by a vortex mixer or an        ultrasonic transducer to thoroughly homogenize the suspension        and breakup any remaining clumps of adhesive;    -   2) A wait of several times the settling time is incurred so all        the microLEDs are collected at the bottom of the container;    -   3) 80 to 90% of the solvent is carefully decanted without        disturbing the settled microLEDs;    -   4) Fresh solvent is added to the container; and,    -   5) Steps 1-4 are repeated.

Typically, three or more cycles of exchange are carried out to ensurethat the adhesive components are removed. The solvent used for adhesiveremoval is chosen entirely based on the ability to dissolve adhesivewithout harming the electrodes of the microLED, so it not always anoptimum choice for subsequent filtering and fluidic assembly operations.The same solvent exchange sequence may be carried out in Step 4 above,substituting a new solvent chosen to optimize cleaning or fluidicassembly. That exchange may also be performed at least three times toensure that the harvest solvent has rinsed the microLEDs off the wallsof the container. After fluid exchange, the microLED solution istransferred from the harvest container to a clean container that can beused to store and transport the suspension. The container should bechemically stable for the suspension solvent and hydrophobic to minimizemicroLEDs sticking to the surfaces. Some suitable materials are acetalhomopolymer, polytetrafluoroethylene (PTFE), polypropylene, polystyrene,and the like.

FIG. 13 is a partial cross-sectional view depicting an appropriatestorage container. Typically, the container is tall and slender withvertical walls and high angle surfaces to minimize accumulation exceptat a single intended location. This control of the accumulation pointallows efficient transfer from the container. If the container istransparent it is convenient to use optical density measurements tomonitor the uniformity of an agitated suspension and to tune themicroLED concentration by adding or removing solvent.

FIG. 14 is a graph showing the major diameter of exemplary microLEDs andcontaminants. The quality of the microLED suspension can be furtherimproved by filtering to remove particles and debris. The exemplarymicroLEDs have a 42 micron (μm) diameter. The small particles belowabout 10 μm can be pieces of electrode metal from the electrode lift-offprocess, fragments of interlevel dielectric (ILD), or airborne dirt.These particles can be captured in the well structures during fluidicassembly, which causes yield loss by interfering with microLED assemblyor bonding between the microLED electrode and the substrate electrode.The large debris can be fragments of GaN or smaller defects encapsulatedin the capture medium that are released in harvest. The filteringprocess removes all the objects which are outside the size band centeredon the diameter of the microLEDs.

It is clear from the figure that broken microLEDs of about the size ofgood microLEDs cannot be removed by filtering, so it is important thatthe selective harvest either captures or removes broken microLEDs beforeharvest. It is also important that suspension handling does not createnew broken microLEDs by excessive mechanical interaction betweenmicroLEDs, or between microLEDs and the containers and fixtures used fortransfer.

A simple filtering method uses mesh filters developed for cellharvesting to produce the desired bandpass around the microLED size.First the suspension is filtered through a 40 μm mesh to remove thelarge debris including any pieces of capture media that escaped thecarrier wafer in harvest. Then the suspension is filtered using a 20 μmmesh to capture the microLEDs and to allow the small particles to passthrough into a waste container. The microLEDs are back-flushed withclean solvent from the filter into a clean container.

FIG. 15 is a schematic drawing of an elution cell. The mesh filteringmethod is cheap and effective but there is significant mechanicalinteraction between the microLEDs and the filter, and in the finefiltering process the devices are subject to shear forces in the pile-upover the filter membrane. More sophisticated filtering methods based onfluid flow are useful to avoid the potential for mechanical damage.Since the hydrodynamic diameter of the microLEDs and the viscosity ofthe suspension solvent are known, it is possible to make an elution cellwhich acts as a filter as shown. The microLED suspension is introducedto the top of the first filter column (μLED supply) and solvent issupplied to the bottom of the column at a rate (Flow 1) that propels allof the particles smaller than a critical size of about 50 μm upward inthe column, while larger particles settle to the bottom of the columnwhere they can be collected and discarded. The small size fraction flowsthrough a transfer channel to the top of the second elution column. Inthe second column Flow 2 is tuned so that particles smaller than about30 μm are forced up and out the waste channel, while the microLEDssettle in the bottom of the column where they can be collected forassembly.

FIG. 16 is a schematic drawing of a continuous flow fractionationfiltering method. The continuous flow fractionation method is similar toan elution cell, using the differences in settling rates of differentparticles to separate good microLEDs from the small particles. As shown,the carrier flow is tuned so that particles smaller than about 30 μmflow out the upper waste gate while the microLEDs, which fall morequickly, flow out the lower port.

Efficient fluidic assembly requires the uniform distribution ofmicroLEDs across the display substrate and the number of microLEDs mustbe sufficient to fill all the available assembly sites (also referred toas trap sites or wells). In practice, the optimum number of microLEDs islarger than the number assembly sites. If the number of microLEDs islower than an optimum value, assembly time increases because microLEDsmust travel farther to reach an empty well site for assembly. However,if the number of microLEDs is higher than the optimum value, the devicestend to cluster together, interfering with the assembly process. Inaddition, all of the excess microLEDs must be removed after assembly, soif too many microLEDs are dispensed, the clean-off time increases andmore microLEDs are included in the recycling process. Therefore, it iscritical that the dispense process is based on a microLED suspensionwhich has a known and well controlled number of microLEDs per unitvolume.

Because variance in the number of microLEDs in an aliquot increases withsuspension concentration and nonuniformity, the concentration ofmicroLEDs in the suspension must be tuned to ensure that the correctnumber of microLEDs are transferred to the display substrate. The numberof microLEDs harvested from the carrier wafer is well determined bycalculating the harvested area with the defect areas removed. Theconcentration of the suspension can then be set simply by adding theappropriate volume of solvent in the final exchange step. However,changes in concentration are caused by solvent evaporation, removingaliquots for dispense, and returning recycled microLEDs to thesuspension, among other things. In order to control concentrationprecisely a system for accurately determining the suspensionconcentration is necessary.

Returning to FIG. 2, for a radially symmetric transparent container,concentration varies only in z-axis, so to quantify microLEDconcentration at several heights in the suspension container, pairedcollimated LEDs (or laser diodes) and photodetectors can be used toshine light through the suspension and measure the density (opticalopacity) which is represented as log(I_(in)/I_(out)). The amount oflight attenuation at various heights in the suspension is directlyproportional to the concentration of microLEDs at those heights, so acalibration curve based on the size of the microLEDs is used to convertthe optical density measurements to a calculated concentration. Opticaldensity measurements are made beginning immediately after agitationproduces a homogenized suspension, to determine the concentration.Measurements versus time and height in the container give the settlingrate directly.

After approximately half the settling time, where detectors 210 a and210 b receive full intensity, detector 210 c returns to an intensityindicating 50-60% of the homogeneous microLED density, and detector 210n sees a microLED density close to that of the homogeneous state. Whenthe suspension has been undisturbed for a long time, compared to thesettling time, all of the microLEDs are collected in the bottom of thecontainer, so light scattering is at a minimum and each of the intensitymeasurements is a maximum. If the suspension is well agitated, microLEDsare uniformly distributed throughout the liquid column and the lightscattering at each height is at a minimum. After agitation stopsmicroLEDs begin to fall under the influence of gravity until they reachthe terminal velocity. With increasing time, the concentration ofmicroLEDs at the top of the fluid column decreases and the detectorintensity increases.

FIGS. 17A through 17C are, respectively, a graph depicting exemplarymeasurements of optical transmittance versus time for a suspension of 42μm diameter microLEDs in 20 mL of isopropanol (IPA), a graph depictingexemplary measurements of optical transmittance versus time for 1.3million microLEDs in different IPA volumes, and a calibration curve formicroLED suspension concentration versus optical transmittance for thissystem. Optical opacity or density is defined herein as the inverse oftransmittance, and the data is divided by the final intensity for thepurpose of normalization. In FIG. 17A approximately 1.2 millionmicroLEDs are suspended in a cylindrical translucent tube 27.5millimeters (mm) in diameter and optical density is measured at fivedifferent vertical positions. At time zero the tube is mechanicallyvibrated to agitate the suspension producing a homogeneous distributionof microLEDs in the fluid column. During agitation, and for the firstfew seconds after agitation, there is significant noise due to airbubbles in the liquid. After agitation the measured light intensity isabout 60% lower than for the settled suspension. The return to fullintensity when all of the microLEDs have settled out of the sensingaperture is a function of the distance from the sensor to the top of thefluid column. So ti is 23 seconds for the upper sensor positioned 12 mmfrom the top of the liquid column, while the bottom sensor at 33 mm fromthe top of the column recovers in 64.5 seconds (t₅). Therefore, theterminal velocity for these microLEDs in IPA is 0.51-0.56 mm/second, sothe settling time for a 45 mm liquid column is about 85 seconds, andsettling after dispensing in a thin layer of liquid takes a few seconds.The transmittance measurement system can also be used to determine thenumber density of microLEDs in a suspension, which is criticalinformation for accurate suspension handling. In FIG. 17B transmittanceis measured as a function of time after agitation for five differentsuspension dilutions. The 42 μm diameter microLEDs were characterized atthe 13 mm position in 20 mL of IPA then measured amounts of liquid wereadded to make suspensions of lower density. In FIG. 17B the initiallight intensity after agitation is lowest for the highest concentration(Ci) and increases with each successive decrease in density. Because theheight of the liquid column increases, the settling time at a fixedposition on the container increases as well.

FIG. 17C is a plot of multiple trials showing a calibration curve thatdetermines the number of microLEDs per milliliter of liquid based on theoptical transmittance of a homogeneous suspension. It can also be seenthat the homogeneity of the suspension is high for several seconds afteragitation and that regime duration decreases with decreasing distance tothe top of the fluid column. It is desirable to draw an aliquot fromthis system a few seconds after agitation and more than 20 mm below thetop surface. This characterization system can be used to select optimumprocess parameters for different microLED sizes, container size andshape, and suspension liquids.

Mixing of the microLED suspension is a balance between the forcenecessary to achieve high suspension uniformity and limiting shear forceto prevent breakage. Mixing can be by external agitation where thesmooth container walls deliver an impulse to the suspension, creatingfluid flow within the suspension container that agitates the microLEDs.

Internal agitation can be by introducing a stream of solvent or gas tothe holding container to induce turbulent liquid flow. Mixing can alsooccur by rapid withdrawal and injection of liquid for example bypipette. The objective of course is to produce a uniform distribution ofmicroLEDs over the vertical column of the container without damaging thedevices.

From the well-mixed suspension, a controlled volume can be drawn outthat contains the number of LEDs intended for one path of dispense. Adispense path can be a single point, a single line segment, a serpentinepath, or some combination of paths. Multiple dispense paths are used toensure complete and uniform dispense over the display assembly area.Because the settling time for LEDs is quite short, particularly in thethin layer of fluid used for assembly, the lateral spread from thedispense path is limited to the scale of millimeters. Therefore, auniform distribution of microLEDs requires multiple dispense pathsrelatively close together.

FIGS. 18A and 18B contrast dispense density gradients, respectively, forsingle and two pass/double speed dispensing paths. The precise method bywhich the dispense volume is transferred from the well-mixed suspensionis determined based on the characteristics of the system and productrequirements: in particular, the assembly substrate dimensions, thedispense head travel speed, the assembly fluid thickness above thesubstrate, the mixing uniformity, and the control over transferredvolumes are all important. These considerations are generally a tradeoffbetween uniformity and cost including system expense, process time, andproduct yield. Deficiencies in dispense uniformity may also becompensated for in the assembly process, so a range of approaches ispresented. For example, a given path can rapidly retrace a single linesegment multiple times to compensate for the nonuniform dispense rate,such as might be caused by settling within the dispense head, andachieve a uniform density along the path as shown in FIG. 18B.

FIGS. 19A through 19C are schematics respectively depicting the directtransfer of a well-mixed suspension from the initial source container tothe assembly substrate via decanting from container, nozzle, and tubing.A transfer of the well-mixed suspension that limits or completely avoidsthe usage of tubing and fittings, may use either direct transfer fromthe source tank or taking discrete aliquots of the source suspension asby a pipette tip. This approach avoids microLED-accumulating dead zonesin fittings and limits interaction with the suspension to surfaces thatare: a) small, b) cleanable, and c) potentially disposable. The pipettetip may draw the suspension from the tank either by volumetricdisplacement (as by a plunger) or by actively applied vacuum. Theadvantage of applying continuous vacuum is that once the aliquotedsuspension is no longer in contact with the source suspension, ambientatmosphere is drawn through the aliquot as bubbles that actively mix thesuspension during transfer.

To limit yield loss, high concentration suspensions are preferred untiljust prior to introduction to the substrate. At this point, the abovealiquoting approach can be coupled with an intermediate, small-volumecontainer where the dispense suspension is supplemented with additionalliquid. This can be done by filling the ink pot with a known volume ofneat liquid through standard systems including tubing, valves, andfittings, because the neat liquid is not subject to the restrictions ofthe microLED suspension. The suspension aliquot can be amended byaspiration of the liquid into the dispense head or by depositing thesuspension into the ink pot, and then withdrawing the mixture into thedispense head. The diluted suspension may then be transferred to thesubstrate.

The above processes for mixing, transfer, and potential dilution aregeneral and their incarnations may be selected to compose a unifiedsystem and method of application that is optimized for the type ofmicroLED display that is being produced. Several full dispense processesare detailed below as examples.

In FIG. 19A, the desired number of LEDs may be dispensed directly fromthe well-mixed container to the assembly substrate. This approach hasthe fewest transfer steps and potentially the fewest surfaces exposed toundesirable microLED stiction. As the fully settled state of thesuspension is nonuniform, the system mixes the suspension to a uniformsuspension density so that volumetric suspension transfer corresponds tomicroLED number transfer. The well-mixed suspension may then be directlytransferred to the substrate. Decanting the microLEDs onto the substratecreates a significant flow throughout the fluid on the substrate, andthis flow can quickly transfer LEDs to the full extent of large areas.However, uniformity may be poor, requiring additional assembly time tocompensate.

In FIG. 19B a small area nozzle, paired with appropriate pressurecontrol in the head space of the container, can transfer suspension in amore controlled manner, although this method is significantly slowerthan decanting. This approach also requires translation of the microLEDcontainer over the assembly substrate with more precision than with thedecanting method.

In FIG. 19C, rather than translate the container, a tube can be usedwith one end submerged in the suspension and the output end translatedover the assembly substrate. The advantage of this approach is that thesuspension container may be actively mixed during dispense, andtranslating the tube end is significantly easier than translating thecontainer. Multiple tubes can originate from one container of microLEDsuspension, significantly increasing the dispense coverage area in asingle pass. The drawback to this method is the extremely high surfacearea that the suspension interacts with, and the resulting capture ofmicroLEDs in the tubing. In applications where cross-contamination isnot a concern, this approach with limited tubing is optimal.

FIG. 20 is a schematic showing controlled volume pipetting from awell-mixed suspension to the assembly substrate. Rather than dispensingdirectly from the container, a pipettor can draw an aliquot from thewell-mixed suspension to dispense directly to the substrate asschematically shown. This approach leverages established techniques inbiological sciences to reliably transfer aliquots with high precision.Additionally, this approach does not require any additionalconfigurations for the suspension container such as pour spouts,nozzles, etc.

The pipette transfer approach is best used when precise volumes and theprevention of cross-contamination are the highest priority, such as whenminimizing excess microLEDs used in assembly, or when sequentiallyassembling distinct size-exclusive microLEDs. The tradeoff is thatpipette dispense is slower than other methods because the pipettor needsto return to the suspension container after each dispense path.Multi-head pipettors exist but they are not well-suited to draw from asingle source, considering the concentration and mixing constraints ofthe suspension container.

FIG. 21 is a schematic showing amended suspension in an intermediatepot, and subsequent aspiration and dispense via a bubble mixed dispensehead. An additional restriction for direct transfer and pipettortransfer methods is that the dispensed volume is equal to the volumetaken from the initial suspension container. To limit the size of thesuspension reservoir for large dispense areas it can be desirable toreduce the concentration of the suspension dispensed on the assemblysubstrate. One way to achieve this dilution is to draw a concentratedaliquot from the suspension supply reservoir and amend it with neatliquid either in the dispense head, or in an intermediate pot as shown.

FIG. 22 is a schematic of a parallel dispense method using an array ofdispense heads. The use of an intermediate pot has a potential advantageof separating the dispense head pickup from the suspension containerpickup. Thus, the system can be scaled up as an array of dispense headspulling from an array of ink pots as shown. The generation of racks ofink pots with a precisely controlled microLED concentration can be donein a separate process, with the racks then loaded into the assembly toolwith the assembly substrates.

The three approaches described are all variations on the central conceptof efficient transfer of microLEDs. Fluidic assembly can be used for awide variety of microLED sizes, assembly areas, and pixel pitches. Someexamples of how variations in assembly requirements may influence theselection of approaches are included below:

For monochrome assembly of small area substrates, direct dispense may bedesirable.

The batch assembly of multiple substrates in parallel from a singlesuspension container suggests the use of a decanting approach.

The serial assembly of substrates is probably best suited to a nozzleapproach.

Large area assembly of large volumes with well-suspended microLEDshaving low batch-to-batch wavelength variation may be most economicalwith a tube-mediated transfer from the suspension source container.

For intermediate scale substrates (a few centimeters on a side) withvaluable microLEDs and low cross-contamination tolerance—such assequentially assembled 3-emitter RGB displays, pipetting dispense may bepreferred. This is especially true in situations where dispense time forthe pipettor is not a significant fraction of the process time.

For very large substrates over Gen 2 size (360×465 mm), single headdispense becomes prohibitively slow and throughput requirements dictatea dispensing system where an array of multiple heads dispense inparallel. For quick-settling suspensions, the ability to vacuum mix thesuspension is important to improving dispense uniformity. Additionally,for large substrates, the total dispense volume requirement becomes veryhigh, and amending the concentrated source suspension improves handlingand mixing uniformity.

Some key requirements for dispensing microLEDs from suspension to asubstrate are limiting microLED waste from breakage, surface adhesion,and application nonuniformity. As such, the suspension optimallyencounters no valves, pumps, or fittings throughout the harvest, filter,mix, dispense, and recycle operations. Unavoidably, there is some lossto the suspension container itself, however thorough rinsing, combinedwith capture and recycling processes, greatly mitigates that loss. Forthe pipette transfer approach, only the pipette tip makes contact withthe suspension. The pipette can be flushed inside-and-out to recovermicroLEDs and can be disposed of to prevent microLEDcross-contamination.

The vacuum-mixed dispense heads, which use disposable tips, as well asthe intermediate ink pots may be flushed for recovery, reused ifcross-contamination is not a concern, or replaced to preventcross-contamination. The embodiment described in FIG. 19C uses tubingthat is unlikely to be cleanable and is presented only as an optionwhere cross-contamination is not a concern and the microLEDs are verywell suspended, minimizing contact with tubing sidewalls.

FIG. 23 is a flowchart illustrating a method for the selective harvestof microLED devices from a carrier substrate. Although the method isdepicted as a sequence of numbered steps for clarity, the numbering doesnot necessarily dictate the order of the steps. It should be understoodthat some of these steps may be skipped, performed in parallel, orperformed without the requirement of maintaining a strict order ofsequence. Generally however, the method follows the numeric order of thedepicted steps. The method starts at Step 2300.

Step 2302 provides inorganic microLED devices attached to a carriersubstrate with an adhesive. In Step 2304 defect regions arepredetermined, where each defect region includes a plurality of adjacentdefective microLED devices or process control structures (e.g., a CMPscratch). Step 2306 forms a solvent-resistant binding material overlyingthe predetermined defect regions. Step 2308 dissolves exposed adhesivewith an adhesive dissolving solvent. Some examples of adhesivedissolving solvent include of acetone, toluene, trichloroethane,N-methylpyrrolidone (NMP), xylene, cyclohexanone, butyl acetate, orcombinations thereof. Step 2310 separates microLED devices locatedoutside the predetermined defect regions from the carrier substrate.Step 2312 maintains the adhesive attachment of microLED devices insidethe predetermined defect regions to the carrier substrate. In responseto separating the microLED devices from the carrier substrate, Step 2314collects functional microLED devices in a harvesting container.

In one aspect, Step 2305 a inspects the carrier substrate to locatedefective microLED devices, and Step 2305 b locates non-predetermineddefect regions including a plurality of adjacent defective microLEDdevices. The inspection process may be performed by optical comparison,electroluminescence, photoluminescence, or cathodoluminesence testing.Then Step 2306 forms the solvent-resistant binding material overlyingthe non-predetermined defect regions. In another aspect, Step 2305 clocates non-predetermined solitary defective microLED devices inresponse to the inspection of Step 2305 a. Then, Step 2307 uses a lasertrimming process to eject the solitary defective microLED devices.

In one aspect, Step 2309 applies an additional motivational force suchas fluid circulation, thermal energy, gravity, vibration, orcombinations thereof, and Step 2310 separates the microLED devices fromthe carrier substrate at least partially in response to the additionalmotivational force.

In one aspect, dissolving exposed adhesive in Step 2308 includesselectively exposing sections of the carrier substrate to the adhesivedissolving solvent. Then, separating microLED devices from the carriersubstrate in Step 2310 includes separating microLED devices from theselectively exposed sections of the carrier substrate. More explicitly,selectively exposing sections of the carrier substrate to the solventmay include the following substeps. Step 2308 a rotates the carriersubstrate in a solvent bath. Step 2308 b exposes a radial section of thecarrier substrate having a radius greater than dto the bath solvent.Then, separating microLED devices from exposed sections of the carriersubstrate in Step 2310 includes separating microLED devices from theradial section of the carrier substrate.

In another aspect, collecting functional microLED devices in theharvesting container in Step 2314 includes replacing the adhesivedissolving solvent with a fluid. If the functional microLED devicescollected in the harvesting container in Step 2314 have an averagecross-sectional physical dimension s, and there are impurities in thefluid, Step 2315 a filters to remove impurities having a maximumcross-sectional physical dimension greater than t, where t>s.Alternatively, or in addition, Step 2315 b filters to remove impuritieshaving a maximum cross-sectional physical dimension less than p, wherep<s. The filtering methods of Step 2315 a and 2315 b may use amechanical mesh, elution, fractionation, or combinations thereof. Forexample, to perform both high-pass and low-pass filtering, mechanicalfiltering may use two different mesh sizes. For elution andfractionation the flow rates need to change, and the two output portsare switched between product and waste. Further, there is no reason touse the same process for both types of filtering. For example, a meshfilter can be used to remove the large contaminants followed by afractionation cell to remove the small particles.

In one aspect, replacing the adhesive dissolving solvent with the fluidin Step 2314 includes exchanging the adhesive dissolving solvent with afiltering solution having a lower viscosity than the adhesive dissolvingsolvent, and the method filters in Step 2315, to remove impurities fromthe filtering solution.

In another aspect, Step 2314 replaces the adhesive dissolving solventwith an assembly solution that may have a lower polarity than theadhesive dissolving solvent or a higher evaporation rate than theadhesive dissolving solvent. Surfactants can also be added, such asanionic, cationic, non-ionic surfactants, or combinations thereof.

FIGS. 24A through 24C are flowcharts illustrating a first method for thedispersal of microLED devices on an emissive display panel. The methodbegins at Step 2400. Step 2402 adds a suspension of harvested microLEDdevices to a transparent first container. Step 2404 agitates thesuspension. Some examples of an agitation process include externalvibration of the first container, creating a fluid flow in thesuspension, and flowing a gas through the first container. Step 2406optically measures the suspension opacity at a plurality of firstcontainer heights. Step 2408 determines suspension homogeneity inresponse to the optical measurements. In response to determining ahomogeneity greater than a homogeneity minimum threshold, Step 2410disperses the suspension on a top surface of an emissive display panel.

In one aspect, Step 2401 a determines the number of harvested microLEDdevices. For example, the number of microLEDs harvested from a carriersubstrate may be known. Step 2409 a calculates the number of microLEDdevices per unit of suspension volume, and dispersing the suspension onthe top surface of the emissive display panel in Step 2410 includesdepositing a known first number of microLED devices in response todispersing a first volume of suspension.

In one aspect, optically measuring the suspension opacity at a pluralityof first container heights in Step 2406 includes substeps. Step 2406 aarranges a plurality of light emitting devices having predeterminedoutput light intensity, directed towards a center axis of the firstcontainer and spaced a first predetermined distance from each otheralong a first vertical axis. Step 2406 b arranges a plurality of lightdetection devices spaced the first predetermined distance from eachother along a second vertical axis, with each light detection devicehaving an input directed towards a corresponding light emitting deviceoutput. Step 2406 c compares the intensities of light received by thelight detection devices.

As an alternative to starting the process with a known number ofmicroLEDs (Step 2401 a), Step 2409 a calculates a first number ofmicroLED devices per unit of suspension volume in response todetermining suspension homogeneity in Step 2408. Step 2412 changes theproportion of fluid-to-LED devices in the suspension by a predeterminedamount of fluid, and Step 2414 optically measures the suspension opacityto calculate a second number of microLED devices per unit of suspensionvolume.

Dispersing the suspension on the emissive display panel in Step 2410includes using one of the following dispersion processes: single-stepmass decantation, multi-step pipette translation, nozzle limitedcontainer translation, and translating tube. The multi-step pipettetransversal dispersal process includes the following substeps. Step 2410a maintains the suspension homogeneity greater than the homogeneityminimum threshold in the first container. Step 2410 b uses a pipette torepeatedly draw a predetermined aliquot volume from the first container.After each aliquot draw, Step 2410 c translates the pipette apredetermined distance with respect to the emissive panel top surface.Step 2410 d releases a predetermined amount of aliquot per second duringthe translation.

In another aspect, the first container is pressure controlled andincludes a nozzle, and the nozzle limited container translationdispersal process includes the following substeps. Step 2410 e maintainsthe suspension homogeneity greater than the homogeneity minimumthreshold in the first container. Step 2410 f translates the firstcontainer a predetermined distance with respect to the emissive paneltop surface, and Step 2410 g releases a predetermined amount ofsuspension per second from the nozzle during the translation.

In one aspect, the first container is pressure controlled and includesan output port connected to one or more delivery tubes, and thetranslating tube dispersal process includes the following substeps. Step2410 h maintains the suspension homogeneity greater than the homogeneityminimum threshold in the first container. Step 2410 i translates thedelivery tube(s) a predetermined distance with respect to the emissivepanel top surface, and Step 2410 j releases a predetermined amount ofsuspension per second from the delivery tube(s) during the translation.

The single-step mass decantation dispersal process includes thefollowing substeps. Step 2410 k maintains the suspension homogeneitygreater than the homogeneity minimum threshold in the first container,and Step 2410 m releases the suspension from the first container onto anemissive panel top surface region using a fixed position center regionrelease or a region translation release.

In one aspect, Step 2401 b determines the number of assembly sites in afirst region of the emissive display panel top surface. Then, dispersingthe suspension on the top surface of the emissive display panel in Step2410 includes depositing a first number of microLED devices at leastequal to the number of assembly sites in the first region.

In another aspect, Step 2409 b determines a number of translating pathiterations for a first region of the emissive display panel, and Step2409 c determines a translation speed. Then, dispersing the suspensionin Step 2410 includes calculating the rate at which the first volume ofsuspension is dispersed in response to the number of path iterations andtranslation speed, to create a uniform density of suspension over theemissive display panel first region.

FIG. 25 is a flowchart illustrating a second method for the dispersal ofmicroLED devices on an emissive display panel. The method begins at Step2500. Step 2502 adds a suspension of harvested microLED devices to atransparent first container. Step 2504 agitates the suspension. Step2506 optically measures the suspension density at a plurality of firstcontainer heights. Step 2508 calculates a first number of microLEDdevices per unit of suspension volume in response to the opticalmeasurements. Step 2510 disperses an aliquot volume of suspension ontoan emissive display panel top surface. Step 2512 repeats the opticalmeasurement, and Step 2514 calculates a second number of microLEDdevices per unit of suspension volume.

In one aspect, Step 2509 a transfers a known aliquot volume ofsuspension to a second container. Step 2509 b modifies the amount offluid in the second container by a predetermined amount, and Step 2509 ccalculates a third number of microLED devices per unit of suspension inthe second container. Step 2509 d disperses the suspension in the secondcontainer onto an emissive display panel top surface.

In another aspect, Step 2516 modifies the amount of suspension fluid inthe first container, and subsequent to agitating the suspension, Step2518 optically measures the suspension density to calculate a fourthnumber of microLED devices per unit of suspension volume.

Systems and methods have been provided for harvesting and dispersingmicroLEDs. Examples of particular process steps and hardware units havebeen presented to illustrate the invention. However, the invention isnot limited to merely these examples. Other variations and embodimentsof the invention will occur to those skilled in the art.

We claim: 1-17. (canceled)
 18. A system for characterizing a micro lightemitting diode (microLED) suspension comprising; a transparent containerhaving a vertical center axis; a plurality of light emitting deviceshaving a predetermined output light intensity, directed towards thecenter axis of the container and spaced a first predetermined distancefrom each other along a vertical first axis parallel to the center axis;a plurality of light detection devices spaced the first predetermineddistance from each other along a vertical second axis parallel to thecenter axis, with each light detection device having an input directedtowards a corresponding light emitting device output and an output toprovide an electrical optical density signal responsive to measuredlight intensity; and, a monitoring device having an input to accept theoptical density signals, the monitoring device comparing the intensitiesof light associated with the optical density signals and supplying anoutput responsive to the comparison.
 19. The system of claim 18 whereinthe container includes a suspension of microLEDs; and, wherein themonitoring device supplies an output selected from the group consistingof a microLED homogeneity measurement and a calculation of a microLEDcount per unit volume of suspension.
 20. The system of claim 19 whereinthe monitoring device includes a non-transitory memory with a storedcalibration curve and supplies a microLED count per unit volume ofsuspension in response to comparing the optical density signals to thecalibration curve.
 21. The system of claim 19 wherein the monitoringdevice has an input to accept a calibration input signal representingthe total number of microLEDs in the suspension and supplies a microLEDcount per unit volume of suspension in response to comparing the opticaldensity signals to the total number of microLEDs.
 22. The system ofclaim 18 wherein the monitoring device accepts sets of optical densitysignals collected over a period of time and supplies an output selectedfrom the group consisting of microLED settling time and microLED size.23. The system of claim 18 wherein the container is divided by aplurality of graduations and includes a homogeneous suspension ofmicroLEDs; wherein the light detection devices detect changes in thelevel of suspension as measured against the container graduations; and,wherein the monitoring device supplies an output selected from the groupconsisting of a number of microLED devices dispersed from the containerand a volume of suspension dispersed from the container.